Electrophotographic processes such as the Carlson process, etc., have been developed with the aim of providing analog depiction depicting of original images. Accordingly, for faithfully reproducing the brightness and darkness of an input light as the brightness and darkness of the corresponding toner images, it has been required that the photoreceptor to be used for this purpose is characterized in that it may pass a photocurrent linearly analogous to the quantity of the input light (the logarithmic value thereof). Therefore, it is essential to select a material having such a characteristic (low .gamma. characteristic) as the material for the photoreceptor.
For this purpose, starting from a material resembling a simple photoconductor in the initial stage of an electrophotographic process, a selenium (Se) series amorphous layer, a silicon (Si) series amorphous layer, a zinc oxide (ZnO)-bonded layer prepared for resembling the Se series amorphous layer, etc., have been used as a photosensitive layer. Furthermore, recently, the electrophotographic technology has developed such that a so-called function-separation type photosensitive layer using, in particular, an organic semiconductor is used.
However, recently, printers and facsimile recording systems have been rapidly converted to an electrophotographic recording system by the combination of the electrophotographic technology and a computer technology. Also, even in an ordinary copying machine, it has been desired to make possible image treatments such as a reversal, cutting, transposing, etc.
For this purpose, it has been desired to change the electrophotographic recording system for a conventional analog recording system for PPC to a digital recording system.
Furthermore, as described above, photosensitizers for photoreceptors used for the traditional electrophotographic process based on the analog concept have low .gamma. characteristics. Accordingly, owing to these characteristics, the photosensitizer is unsuitable for an electrophotograph necessary for depicting inputted digital light signals, such as printers for data output of a computer, digital copies obtained by digital processing images, as digital images. That is, this is because the photoreceptors using the photosensitizers faithfully depict even the deterioration of the digital signals in a signal route from a computer or an image processor to the electrophotographic devices and also the aberration by an optical system for focussing a light beam for writing or for imaging original images and thus the original digital images cannot be reproduced.
Also, as the light source to be used in such a digital recording system, there are gas lasers such as an argon laser, a helium-neon laser, etc., a semiconductor laser, a light emitting diode, etc. From among these lasers, the semiconductor laser is being widely used at present due to its suitable size and reasonable cost, and it is desirable that a photoreceptor also has a high sensitivity to the light emission wavelengths (at about near infrared) of the semiconductor laser.
That is, it is required that the photoreceptor to be used for the digital recording system have characteristics (hereinafter, are referred to as high .gamma. characteristics) that the photocurrent linearly analogous to the input light quantity is not passed therethrough and the photocurrent is passed in a small amount until a certain quantity of light is reached but when the quantity of light exceeds a definite value, the photocurrent passes rapidly until the potential reaches the base potential (residual potential).
Accordingly, to provide of a material for photoreceptor having a high sensitivity and the high .gamma. characteristics, which can be utilized in the field, has been strongly desired.
Under these circumstances, a concept of a photoreceptor for digital light input is disclosed in JP-A-1-169454 (the term "JP-A" as used herein means an "Japanese Patent Laid-Open Publication"). However, there is no practical description of the materials which can be used for the photoreceptors for digital light input.
Also, even titanyl phthalocyanine described in JP-A-3-37662 and the phthalocyanine mixed crystal described in JP-A-2-84661 are still insufficient for use as the photosensitizers for the photoreceptors for digital light input described above since these .gamma. characteristics are not so high and these residual potentials are high.